1. Field of the Invention
This invention relates to imaging of subterranean formations with logging tools, and more particularly to the use of nuclear quadrupole resonance phenomena for ascertaining properties of subterranean materials.
2. Background of the Related Art
As the aggressive search for various minerals and geologic deposits continues unabated, the techniques for subterranean exploration have become increasingly sophisticated. Improved search capabilities requires improvements in the abilities to evaluate materials such as formation fluids in-situ.
Nuclear Quadrupole Resonance (NQR) is a well known technique for identifying and evaluating materials. Nuclear Quadrupole Resonance (NQR) is a phenomenon in the general class of magnetic resonance. Like nuclear magnetic resonance (NMR) and electron spin resonance (ESR), NQR makes use of a resonant exchange of energy between the spin of each nuclei and the environment. This exchange can be detected and used to estimate the properties of the spin-environment interaction and therefore obtain information about the environment in which the nuclei are located. Like NMR and ESR, in NQR the resonant exchange uses a radio-frequency magnetic field. However, unlike NMR and ESR, the resonance condition for NQR does not require an external magnetic field. Rather, the energy of any particular spin state depends on the electric field gradient at the site of the spin. While NMR and ESR depend on magnetic interactions with the magnetic dipole moment of the spin, NQR depends on interactions with both the magnetic dipole and the electric quadrupole moments of the spin. For this reason, NQR phenomena are more complicated to interpret than NMR. Accordingly, use of NQR has not been explored nearly to the extent of NMR.
In recent years, the possibility of using NQR for detection of chemical explosives has been explored. Research in this respect has been sparked by the proliferation of plastic anti-personnel mines. Current techniques for explosives detection relies upon known characteristics of typical explosives. For example, Nitrogen is a common chemical component of explosives. Conveniently for NQR measurements, nitrogen-14 (N-14) has a nuclear spin of one (1) and is almost 100% abundant. Explosives detection using NQR typically calls for use of a radiofrequency (RF) surface coil to detect the NQR resonance of the 14N nuclei. The resonant frequency obtained can indicate whether 14N nuclei are in an explosive compound and possibly what kind of explosive is involved. The amplitude of the resonance can indicate the amount of explosive as well.
Imaging of materials is also possible with NQR. In fact, NQR imaging has some advantages over NMR (at least for some materials) because the NQR resonant lines in zero field are more narrow than the resonant lines using NMR at high field.
The principle method used in NQR imaging is derived from rotating-frame zeugmatography. (Hoult 1979). In this method, an RF field gradient is applied to a target volume and the location of the spins is encoded either in the phase of the signal or the amplitude. Images using rotating-frame NQR imaging techniques have been made of such diverse materials such as arsenolite (75As), copper oxide (63Cu) and boric acid (11B). A summary of NQR imaging can be found in various references.
Some geological applications of NQR have already been suggested (reference may be had to Marino, Wenk et al. 1980; Schempp, Klainer et al. 1980; and Schempp, Murdoch et al. 1981). These applications include in-situ estimates of stress, in-situ elemental analysis of minerals, and characterization of phase transitions in minerals. However, no methods for accomplishing these suggestions have been developed as yet.
Elemental analysis of minerals is possible because the resonance frequency of the NQR signal is highly dependent on the quadrupole coupling constant (QCC) of the probe nucleus with the mineral lattice. Different elements will have different resonant frequencies. In fact, isotopes of the same element can have different frequencies even when residing in identical crystal sites.
One element of particular importance to geologists is aluminum. Aluminum-27 (27Al) has a spin of 5/2 and is 100% abundant. In a pure sandstone reservoir, the amount of aluminum is directly related to the reservoir's clay content. However, in reservoirs with many mineral types, estimates of the aluminum content might be reduced, with a caveat that the resonant frequency of the aluminum also depends in which mineral the atoms of aluminum reside. With this in mind, it may be possible not only to estimate that total amount of aluminum in the reservoir but also estimate mineral content in situ by an analysis of NQR spectra.
In principle, one could also measure NQR spectra for other isotopes such as 35Cl, 23Na, 25Mg, and 43Ca to obtain detailed information regarding the mineral composition of subterranean materials, including rock and fluids.
What are needed are techniques for applying NQR technologies to subterranean exploration.